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The Web Shield in Avast Antivirus monitors your Internet activity and blocks URLs of known dangerous sites before your computer loads them. This prevents any risk of infection from these sites. A malicious URL warning will usually interrupt you a single time, and then go away. If Avast continually displays the same warning, take steps to ensure that your system is not already compromised and double-check your Internet activity.

Legitimate Blocks

If you're visiting the same website each time the warning appears, the site may actually contain dangerous content. Check an online reputation site, such as Web of Trust's Safe Browsing Tool, for the reputation of the page you were using. Even if you have the warnings pop up on sites you know to be safe, these sites may have dangerous content displaying inside advertisements without the site owner's knowledge. Writing to the site administrator with your warning information will help them track down the issue.

Fake Warnings

Many malware programs install fake antivirus software onto your computer. These programs display warnings that mimic real antivirus pop-ups in an attempt to extort customers for a solution. Some of these fake antivirus programs use the Avast name to appear legitimate. If you aren't sure whether your warning is real, try uninstalling the program, downloading a safe copy from Avast's official website and reinstalling it. After the new install, run a full virus scan.

Existing Infections

A blocked URL warning that shows up regularly can point to an existing virus on your computer. The virus will repeatedly attempt to connect back to a dangerous server, which prompts the block, but the block alone does not fix the infection. Run a full scan in Avast, and perform a complete malware scan with another product, such as Malwarebytes Anti-malware, Spybot S&D or Ad-Aware. Repair any problems these programs locate.

Finding Further Assistance

If no other methods solve the problem, you likely have a deeply-ingrained virus that Avast can't fix automatically. The free utility HijackThis will provide a detailed analysis of all processes running on your system. Though this analysis is difficult to understand by the average user, the information, along with your Avast logs, will help a professional eliminate even stubborn infections. Run the scan and provide the result to a local IT expert or post it on a computer security Web forum asking for assistance. Avast's own forum has many users willing to help deal with recurring problems.

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About the Author

Aaron Parson has been writing about electronics, software and games since 2006, contributing to several technology websites and working with NewsHour Productions. Parson holds a Bachelor of Arts from The Evergreen State College in Olympia, Wash.